Field of the Invention
The invention relates to the field of spectroscopy including, for example, Fourier transform spectroscopy.
Description of the Related Art
Fourier transform spectroscopy is a technique that can be used for obtaining information about the spectral content of light from a source. Many Fourier transform spectrometers (FTS) employ a Michelson interferometer and measure the spectrum of light that is encoded in a time-varying signal that results from the interaction of the input light with the interferometer. In a Michelson FTS the interference pattern is sampled in time. The Michelson FTS uses a moving mirror that causes an input beam, which is split into two arms and then recombined, to experience a time varying optical path difference (OPD) between the two arms. Illuminated by monochromatic light, the detector response to this time varying OPD is a sinusoidal signal whose period is a function of the rate of change of the OPD and the wavelength of the incident light. The wavelength of the incident light is recovered from the sampled signal by precise knowledge of the rate of change of the OPD, usually using a reference laser signal. Illumination by multiple wavelengths produces a resultant pattern that is additive; the intensities of the individual wavelengths are recovered using the Fourier transform after appropriate preprocessing. The transformation from sampled interference pattern (i.e., the interferogram) to spectrum is well-established.
Another type of FTS is the spatial FTS, where the spectrum of the input light is encoded in a spatial pattern sampled by a detector array. A spatial FTS may use optics to produce a gradient in OPD across a detector array, for example, by slight deviations of mirrors or beamsplitters relative to perfect symmetry. The interaction of illuminating light with this gradient in OPD produces an interference pattern that is sampled by the array. The interferogram is calibrated in wavelength (i.e., the slope of the OPD is determined) using a known monochromatic source (e.g., light transmitted through an interference filter). Once sampled and corrected for non-uniformities in response of the detector array elements, data processing can be similar to the Michelson FTS data processing.